1. Field of the Invention
The present invention relates to a developing device utilizing a one-component developer containing magnetic toner.
2. Description of the Prior Art
In the development with a one-component developer (hereinafter also called toner) there is preferred the use of an insulating magnetic developer in order to satisfy the requirement of image transfer onto a plain paper. Also in case such insulating magnetic developer is charged at the development of a latent image there are required various factors for stabilizing the developing density and the developed image. The absence of coagulation of the developer is one of such requirements. For meeting such requirement preferred is the use of a developing method in which the developer applied on developer supporting means is charged and positioned separate from latent image bearing means, whereby the developer supported on said supporting means jumps up to the image area of the latent image. This method, effective for preventing compression and coagulation of the developer between the developer supporting means and the latent image bearing means and known as toner transfer development as disclosed in the U.S. Pat. Nos. 3,232,190 and 2,839,400, utilizes non-magnetic toner. There is also known a developing method in which an electric field of a high frequency higher than 1.5 kHz is applied between the developer supporting means and the rear face of the latent image bearing means, as disclosed in the U.S. Pat. Nos. 3,866,574 and 3,893,418. Despite of various trials, however, it has been difficult to realize a stable developing density and to constantly maintain a uniform developer layer on the developer supporting means without the coagulation of the developer.
In order to resolve such difficulties there are already proposed other developing methods different from that disclosed in the above-mentioned United States Patents. For example the United States Patent Applications Ser. Nos. 938,101, filed Aug. 30, 1978; 938,494, filed Aug. 31, 1978; 58,434, filed July 17, 1979 and 58,435, filed July 18, 1979, U.S. Pat. No. 4,292,387 of the present assignee disclose a developing method allowing the toner transfer onto a plain paper with the use of insulating toner particles incorporating magnetic powder therein, wherein a magnetic blade positioned in the vicinity of the developer supporting means constantly and stably maintains a uniform developer layer on said supporting means, thus preventing the coagulation of the developer and enabling satisfactory image formation with stable developing density. More specifically a magnet roll is positioned in a non-magnetic developing sleeve, and a toner layer defining blade made of a magnet or a magnetic material is positioned corresponding to a magnetic pole of said magnet roll and spaced from said developing sleeve to define the thickness of the toner layer supported on said developing sleeve. The other magnetic pole of the magnetic roll is fixed in a position facing the latent image bearing means in the developing zone, and the image development is effected by rotating said developing sleeve in a same direction and at a substantially same peripheral speed with the latent image bearing means and in such a manner that said developing sleeve is spaced from said latent image bearing means by a distance larger than the thickness of said toner layer.
In such development an appropriate intensity of the developing electrode is important as it has a significant effect on the image quality, in contrast to the dry development with two-component toner in which the image quality is not essentially affected by the intensity of the developing electrode of the fixed magnet roll positioned in the rotary developing sleeve so as to face the developing zone. For example the developing function may be disabled if the developing electrode is excessively strong. Consequently in the development with one-component magnetic toner, an appropriate intensity of the developing electrode is essential for obtaining a satisfactory developed image.
On the other hand, as explained in the foregoing, the one-component toner generally tends to coagulate, thus resulting in various troubles. This tendency becomes stronger with the increase in the resistance of the toner, and is particularly marked in the insulating toner which is to be charged in the development. Such toner showing the coagulating tendency develops increasing coagulation when subjected to external force in the developing unit, thus hindering smooth transportation of the toner along with the rotating sleeve, eventually forming toner pool or disabling the developing function.
The developing device as explained in the foregoing is generally provided with elastic members, such as felt or moltprene, maintained in pressure contact with the lateral end portions of the developing sleeve for preventing the toner leaking beyond said lateral ends. In such structure, however, the toner particles are in a state of easily causing the coagulation because of the friction with said elastic members. Consequently the toner transporting force caused by the sleeve rotation is inevitably weakened, and there is easily generated a toner pool, which progressively develops in size, eventually causing toner leak from the developing device or direct toner contact with the latent image bearing means. Thus, in a developing device utilizing one-component magnetic toner a satisfactory image development may be hindered by such toner pool formation even if the intensity of the developing electrode is appropriately selected.